Add buildpack support for Heroku-26 (#2073)

Following on from #2056 (which added support for building
and releasing Python binaries for Heroku-26), this adds support
for the new stack to the buildpack itself.

Note: Since Ubuntu 26.04 isn't GA yet, Heroku-26 is currently
only available on Heroku for use by internal users.

GUS-W-20775616.

Author: edmorley

.github/workflows/ci.yml

@@ -39,7 +39,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        stack: ["heroku-22", "heroku-24"]
+        stack: ["heroku-22", "heroku-24", "heroku-26"]
     env:
       HATCHET_APP_LIMIT: 300
       HATCHET_DEFAULT_STACK: ${{ matrix.stack }}

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Added support for Heroku-26. ([#2073](https://github.com/heroku/heroku-buildpack-python/pull/2073))
 
 ## [v341] - 2026-04-07
 

Makefile

@@ -1,7 +1,7 @@
 # These targets are not files
 .PHONY: lint lint-scripts lint-ruby check-format format run publish
 
-STACK ?= heroku-24
+STACK ?= heroku-26
 FIXTURE ?= spec/fixtures/python_version_unspecified
 # Allow overriding the exit code in CI, so we can test bin/report works for failing builds.
 COMPILE_FAILURE_EXIT_CODE ?= 1

lib/checks.sh

@@ -4,7 +4,7 @@ function checks::ensure_supported_stack() {
 	local stack="${1}"
 
 	case "${stack}" in
-		heroku-22 | heroku-24)
+		heroku-22 | heroku-24 | heroku-26)
 			return 0
 			;;
 		cedar* | heroku-16 | heroku-18 | heroku-20)

spec/fixtures/python_version_outdated/.python-version

@@ -1 +1 @@
-3.13.2
+3.14.0

spec/hatchet/pipenv_spec.rb

@@ -455,7 +455,10 @@
     end
   end
 
-  context 'when the Pipenv and Python versions have changed since the last build' do
+  # TODO: Enable on Heroku-26 after the Pipenv and default Python versions next change,
+  # since for now there isn't a historic buildpack version we can use in this test whose
+  # stack check permits Heroku-26 and that also uses older Pipenv/Python versions.
+  context 'when the Pipenv and Python versions have changed since the last build', stacks: %w[heroku-22 heroku-24] do
     let(:buildpacks) { ['https://github.com/heroku/heroku-buildpack-python#v313'] }
     let(:app) { Hatchet::Runner.new('spec/fixtures/pipenv_basic', buildpacks:) }
 

spec/hatchet/poetry_spec.rb

@@ -131,7 +131,10 @@
     end
   end
 
-  context 'when the Poetry and Python versions have changed since the last build' do
+  # TODO: Enable on Heroku-26 after the Poetry and default Python versions next change,
+  # since for now there isn't a historic buildpack version we can use in this test whose
+  # stack check permits Heroku-26 and that also uses older Poetry/Python versions.
+  context 'when the Poetry and Python versions have changed since the last build', stacks: %w[heroku-22 heroku-24] do
     let(:buildpacks) { ['https://github.com/heroku/heroku-buildpack-python#v313'] }
     let(:app) { Hatchet::Runner.new('spec/fixtures/poetry_basic', buildpacks:) }
 

spec/hatchet/python_version_spec.rb

@@ -88,17 +88,21 @@
       end
     end
 
-    context 'with an app last built using an older default Python version' do
+    # TODO: Enable on Heroku-26 after the default Python version next changes (for the 3.14.5
+    # release), since for now there isn't a historic buildpack version we can use in this test
+    # whose stack check permits Heroku-26 and also has a different default Python version so
+    # that we can test the sticky versions feature.
+    context 'with an app last built using an older default Python version', stacks: %w[heroku-22 heroku-24] do
       # This test performs an initial build using an older buildpack version, followed
       # by a build using the current version. This ensures that:
       # - The current buildpack can successfully read the version metadata
       #   written to the build cache by older buildpack versions.
       # - If no Python version is specified, the same major version as the
-      #   last build is used (sticky versioning).
+      #   last build is used (sticky major versioning).
       # - Changes in the pip version are handled correctly.
       let(:buildpacks) { ['https://github.com/heroku/heroku-buildpack-python#v267'] }
 
-      it 'builds with the same Python version as the last build' do
+      it 'builds with the same major Python version as the last build' do
         app.deploy do |app|
           update_buildpacks(app, [:default])
           app.commit!
@@ -566,25 +570,25 @@
       app.deploy do |app|
         expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
-          remote: -----> Using Python 3.13.2 specified in .python-version
+          remote: -----> Using Python 3.14.0 specified in .python-version
           remote: 
           remote:  !     Warning: A Python patch update is available!
           remote:  !     
-          remote:  !     Your app is using Python 3.13.2, however, there is a newer
-          remote:  !     patch release of Python 3.13 available: #{LATEST_PYTHON_3_13}
+          remote:  !     Your app is using Python 3.14.0, however, there is a newer
+          remote:  !     patch release of Python 3.14 available: #{LATEST_PYTHON_3_14}
           remote:  !     
           remote:  !     It is important to always use the latest patch version of
           remote:  !     Python to keep your app secure.
           remote:  !     
           remote:  !     Update your .python-version file to use the new version.
           remote:  !     
           remote:  !     We strongly recommend that you don't pin your app to an
-          remote:  !     exact Python version such as 3.13.2, and instead only specify
-          remote:  !     the major Python version of 3.13 in your .python-version file.
+          remote:  !     exact Python version such as 3.14.0, and instead only specify
+          remote:  !     the major Python version of 3.14 in your .python-version file.
           remote:  !     This will allow your app to receive the latest available Python
           remote:  !     patch version automatically and prevent this warning.
           remote: 
-          remote: -----> Installing Python 3.13.2
+          remote: -----> Installing Python 3.14.0
           remote: -----> Installing pip #{PIP_VERSION}
         OUTPUT
       end

spec/hatchet/stack_spec.rb

@@ -3,7 +3,7 @@
 require_relative '../spec_helper'
 
 RSpec.describe 'Stack changes' do
-  context 'when the stack is upgraded from Heroku-22 to Heroku-24', stacks: %w[heroku-22] do
+  context 'when the stack is upgraded from Heroku-24 to Heroku-26', stacks: %w[heroku-24] do
     # This test performs an initial build using an older buildpack version, followed by a build
     # using the current version. This ensures that the current buildpack can successfully read
     # the stack metadata written to the build cache in the past. The buildpack version chosen is
@@ -14,8 +14,8 @@
 
     it 'clears the cache before installing again whilst preserving the sticky Python version' do
       app.deploy do |app|
-        expect(app.output).to include('Building on the Heroku-22 stack')
-        app.update_stack('heroku-24')
+        expect(app.output).to include('Building on the Heroku-24 stack')
+        app.update_stack('heroku-26')
         update_buildpacks(app, [:default])
         app.commit!
         app.push!
@@ -64,7 +64,7 @@
           remote:  !     file and this warning will be made an error.
           remote: 
           remote: -----> Discarding cache since:
-          remote:        - The stack has changed from heroku-22 to heroku-24
+          remote:        - The stack has changed from heroku-24 to heroku-26
           remote:        - The Python version has changed from 3.12.3 to #{LATEST_PYTHON_3_12}
           remote:        - The buildpack cache format has changed
           remote:        - The legacy SQLite3 headers and CLI binary need to be uninstalled
@@ -77,20 +77,20 @@
     end
   end
 
-  context 'when the stack is downgraded from Heroku-24 to Heroku-22', stacks: %w[heroku-24] do
+  context 'when the stack is downgraded from Heroku-26 to Heroku-24', stacks: %w[heroku-26] do
     let(:app) { Hatchet::Runner.new('spec/fixtures/python_3.14') }
 
     it 'clears the cache before installing again' do
       app.deploy do |app|
-        expect(app.output).to include('Building on the Heroku-24 stack')
-        app.update_stack('heroku-22')
+        expect(app.output).to include('Building on the Heroku-26 stack')
+        app.update_stack('heroku-24')
         app.commit!
         app.push!
         expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
           remote: -----> Using Python 3.14 specified in .python-version
           remote: -----> Discarding cache since:
-          remote:        - The stack has changed from heroku-24 to heroku-22
+          remote:        - The stack has changed from heroku-26 to heroku-24
           remote: -----> Installing Python #{LATEST_PYTHON_3_14}
           remote: -----> Installing pip #{PIP_VERSION}
           remote: -----> Installing dependencies using 'pip install -r requirements.txt'

spec/hatchet/uv_spec.rb

@@ -128,7 +128,10 @@
     end
   end
 
-  context 'when the uv and Python versions have changed since the last build' do
+  # TODO: Enable on Heroku-26 after the uv and default Python versions next change,
+  # since for now there isn't a historic buildpack version we can use in this test
+  # whose stack check permits Heroku-26 and that also uses older uv/Python versions.
+  context 'when the uv and Python versions have changed since the last build', stacks: %w[heroku-22 heroku-24] do
     let(:buildpacks) { ['https://github.com/heroku/heroku-buildpack-python#v313'] }
     let(:app) { Hatchet::Runner.new('spec/fixtures/uv_basic', buildpacks:) }